Fun with bananas:
Okay, this takes some prepartion but the looks on people's faces are worth it. Take a ripe banana and a straight pin. Poke the the pin into the banana and wiggle it side to side. Then move the pin down a little and do it again. Make the same kind of cutting motions with the pin. I usually make about twenty sticks/cuts all the way down the banana.
Now find your mark. A kid say, and tell him that you've bought a new kind of banana, one that comes already sliced. Have him look at the banana closely, (If you make all the pins holes along one line they are invisible.)
Wait till your mark says, "aw shucks or you're cracked", then carefully peel the banana revealing the beautiful slices you made.